In the prior art, for instance, as disclosed in the respective specifications and drawings of Laid-Open Japanese Patent Specifications 60-21225 (1985) and 60-110419 (1985), a molding method by means of a compression molding method is achieved through the steps of pouring resin material to be molded into a heated metal mold, actuating the molding machine to press the material at a low pressure, after the molten resin material to be molded has been filled within the metal mold, once opening upper and lower metal molds a little to remove a gaseous component in the material, again pressing the material at a high pressure, continuing the pressing and the heating until the material has finished to be hardened, and thereafter taking out a molded ariticle by opening the mold. On the other hand, in injection-compression molding, when molten resin material is injected into a metal mold, an injection flow rate and an injection flow velocity of the molten resin are controlled by a moving speed of a screw and a pressing force of the screw.
However, in the above-described compression molding method in the prior art, due to the fact that the pressing is effected as divided into low-pressure pressing and high-pressure pressing, a lot of man-hours are necessitated.
In addition, in the injection-compression molding, since molten resin is injected into a metal mold as varying a speed and a pressure of a screw, depending upon viscosity and elasticity of the molten resin, the responsibility would become an issue, and furthermore, in molding at a low flow resistance such as injection-compression molding, a time of injecting molten resin into a metal mold or an amount of molten resin to be molded cannot be stabilized, so that defects (short-sits, shrinks and warps) would arrive in the molded articles.
Moreover, in the case where compression is commenced during injection, a flow resistance of molten resin would increase gradually, hence in the injection method in the prior art, an amount of injection would vary depending upon viscosity and elasticity of the molten resin, resulting in surplus or shortage in the amount of injection, and so, there is a shortcoming that burrs or short-sits are generated.